Turbocharging carbureted petrol 2 stroke engines

[bluechipx]

I'm an old school machinist, sine plates, rotary tables and such. When I was buying my first Hurco machining center when the owner of the shop I was dealing with stepped away, I asked the twenty some year old operator how difficult it was to learn how to operate it. He said he could teach a monkey to do it in a half hour which gave me confidence. Six months later I was still struggling nand could only do the basics that could be done from the machines console. When it gets into 3-D I'm lost. My younger brother uses one of the mastercams from a computor and sends it to the machine. I did take an old crankcase and cut it across the area where the connecting rod travels and duplicated the shape on a piece of graphite and EDM'ed the pockets (which aren't half rounds , but a somewhat unique shape). One day last fall I wanted to try out a EGT gauge when it was in the thirties and snowed the next day. I took it kinda easy, probably around 80 mph and had the strange feeling the hydro was getting a little high in the front so I didn't look at my gauge and kept my eyes on the river. The trim adjustment jammed into a position that caused way to much bow lift. A professional racer friend saw the picture and said 1/4" more bow lift and it surely would have blown over. It's fixed now and it should be perfect in the spring when I run it again.

 

[CLatch]

I'm an old school machinist, sine plates, rotary tables and such. When I was buying my first Hurco machining center when the owner of the shop I was dealing with stepped away, I asked the twenty some year old operator how difficult it was to learn how to operate it. He said he could teach a monkey to do it in a half hour which gave me confidence. Six months later I was still struggling nand could only do the basics that could be done from the machines console. When it gets into 3-D I'm lost. My younger brother uses one of the mastercams from a computor and sends it to the machine. I did take an old crankcase and cut it across the area where the connecting rod travels and duplicated the shape on a piece of graphite and EDM'ed the pockets (which aren't half rounds , but a somewhat unique shape). One day last fall I wanted to try out a EGT gauge when it was in the thirties and snowed the next day. I took it kinda easy, probably around 80 mph and had the strange feeling the hydro was getting a little high in the front so I didn't look at my gauge and kept my eyes on the river. The trim adjustment jammed into a position that caused way to much bow lift. A professional racer friend saw the picture and said 1/4" more bow lift and it surely would have blown over. It's fixed now and it should be perfect in the spring when I run it again.

Well I'm a welder/fabricator by trade, not anywhere close to a machinist, only thing I can operate like that is a drill press lol. I was asking so I could possibly have the engineering guys at work do their magic and recreate that idea in 6 cyl form, because I know for a fact that reeds won't survive boost, and they're INSIDE the crack case so it makes it that much harder. I really do like the idea of easier access to Reeds and a different type being that there's only 1 oem style aftermarket Reed option for the towers, and it's Boyosen. And yes, looking at that picture looks like it wasnt far off from tipping! I bet it sounds amazing tho! I do however have a question about cooling.
Did you have to modify the factory cooling on the block at all? And how about the lower unit?

 

[bluechipx]

No cooling mods at all. In fact the one to one gear case off a 45SS johnson race engine (stronger than the "D" Merc) had such a big water pump in it the engine wouldn't warm up so I had to re-route some of the water back to the river to get sensible engine temp. I really hope you aren't planning on using the stock gear ratio fishing lower unit gear box. Instead use one of the speedmaster series units. When a new guy would get a hydro, on a budget, and try high pitch props on the fishing unit, a fifty hp Merc (44 cu in) would barely hit fifty. A simple change to a "quicky" or one to one racing style lower unit would jump the speed easily to 76-78 on the first try.

 

[bluechipx]

BTW, in order to match the block to the new crankcase and wind up with the crank bore being round and sized right I did the bore on a DeVlieg jig mill. Yours being 50% longer will be a little tougher. I could be wrong but I think when the engineering guys see what you are asking they won't be quick to agree, it's quite a job from huge block of aluminum into finish product. Let me know how it goes when you run it past them.

 

[CLatch]

BTW, in order to match the block to the new crankcase and wind up with the crank bore being round and sized right I did the bore on a DeVlieg jig mill. Yours being 50% longer will be a little tougher. I could be wrong but I think when the engineering guys see what you are asking they won't be quick to agree, it's quite a job from huge block of aluminum into finish product. Let me know how it goes when you run it past them.

I'm aware about the lower, was just curious of what kind you had in your pictures. I might use it for initial testing purposes but not permanently. Im not really familiar with outboard racing or racing products for outboards in general, this project will just be a hobby deal for me.
And Hopefully the engineers and machinists won't turn it down, I mean I do work for one of the top automotive blower companies in the US, and we have experimented with outboard stuff a while back but ended up sticking to inboard blowers due to market not being there. I doubt it was anything this old, but I also doubt the guys will be scared to do it.
Also, what size carburetors did you use? Going to try to get an idea of the amount venturi I need along with the jet sizes you used. I imagine a good starting point would be slightly bigger carb than you used at 3 total?

 

[bluechipx]

In case anyone is still following this thread, I did a real stupid thing last fall and nearly died. I have a formula/champ style one person tunnel hydro with a moderate v6 2.5 liter mercury on it. They all have power trim and trimming the engine angle is critical for controlling the bow lift.

I was just going to go for a easy ride and never even told anyone I was doing it. It was just going to be a easy run so I wasn't even wearing a helmit (I know, stupidity!). I had the trim set very conservatively and gradually increased my speed as I was going from river to lake in near calm wind. A recovered and dried gps read 118 mph. At somewhere around 118 mph the bow lifted so rapidly that there was no time to trim out of it.

All I could see was sky when I tightly closed my eyes and put a deathgrip on the steering wheel. I knew this was going to be a bad one. After a few seconds I felt a hard jarring and opened my eyes to see I landed upright in the water, unhurt and not even wet! A quick inspection showed the rear cowling gone and a badly leaking gas tank which broke loose. I stopped the leak by tipping the tank and was afraid to disconnect any electrical contacts to prevent sparking and igniting an inch or more of gas in the entire bottom of the hydro.

I removed my shirt and soaked up the gas like a sponge. Apparently I did a complete 360 and landed in a headache producing thud. It was one hour before a friend in a jetski came to tow me back. Not one other boat came by in that hour.

I have never given up on any project but this time I am. The boat is shrink wrapped and I'll never drive it again. In fact I'll destroy it before I sell it unless a person can do some real convincing that they have a ton of experiance and I don't have to hear they either died or were badly hurt in it. Google 'tunnel boat blowovers' for some footage of accidents others had and you will see how lucky I was to walk away unhurt but somewhat smarter!

 

[Tom.G]

Sounds like a case of "Live and Learn."
Too often the first requirement is skipped.

 

[Kaseytoo]

When a 4 stroke engine is converted to 2 stroke, the cylinders are used at twice the rate, so it has an improved power to weight ratio, but with higher emissions. A 4 stroke engine has separate inlet and exhaust manifolds so it can be easily turbocharged.

For inducted air-fuel mix engines the maximum compression ratio is limited by the octane rating of the fuel. The effective compression ratio is the product of the cylinder compression ratio, and the air pressure boost ratio. Most engines without superchargers are designed for optimum compression ratio, so there is little room for improvement. You might need to change from gasoline to ethanol for a higher compression ratio.

Fuel injection overcomes the compression limit. There have been many fuel injected two stroke diesels with superchargers.

I have built a small 170 cc split single 2 stroke engine. I'd like now to supercharge it. Can you recommend what size and type I could use?

 

[Baluncore]

I have built a small 170 cc split single 2 stroke engine. I'd like now to supercharge it. Can you recommend what size and type I could use?

Firstly, piston material and so piston temperature will decide the boost limit. With two pistons close together, cooling is more difficult than single-cylinder engines with air-cooling, or multicylinder engines with water jackets.

Secondly, with higher cylinder pressures, the main bearings will need improved lubrication. That may be difficult if you pass pressure boosted air-fuel mix through the crankcase. Maybe you could charge the inlet port directly from the supercharger, SC.

The required SC capacity will depend on the optimum engine RPM, and the required boost. Estimate the boost factor. If the SC is driven by the 170 cc engine, then boost ratio * 170 cc * RPM = capacity in cc/min. Select an appropriate gear ratio for the SC.

 

[Kaseytoo]

Thanks for the reply. I'm hoping that with internal oiling, the exhaust piston can have direct oil cooling to cover that. ports will be direct from the s/charger and or then turbo if possble. The a radicall split single design offers opportunities not found in normal two strokes. (see att. as a possibility)
Peak revs can be to 5000 rpm when needed. It needs 850000 ccs per minute at that rpm. some roots types (300 and 500) models are available from China. would one of them be adequate. Mazda use one type with their new valved 2 stroke.

 

[Baluncore]

Aircraft offered better piston cooling opportunities than did static engines. Air cooling of opposed pistons can be improved over split cylinders. The split two-stroke, evolved into the opposed piston two-stroke aero-engine during the 1930s.
That then became the Napier Deltic engine in the 1950s, used in ships and trains.

some roots types (300 and 500) models are available from China. would one of them be adequate.

Those superchargers are low-cost, and have a capacity greater than required for your engine. However, the following description suggests (in its last line), that a turbocharger, rather than a blower, might be more efficient.
https://en.wikipedia.org/wiki/Junkers_Jumo_205#Design_and_development

 

[Kaseytoo]

I see the only connection with an O.P. engine is the use of a common combustion chamber to drive both pistons equally.. Both have problems keeping the exhaust piston cool, and the O.P. engine has better cylinder charge filling. The split single stops any escape of charge thru the exhaust ports.>better economy at low revs.. Charging continues after the Ex.ports have shut. Adequate fuel charging is its main problem, but solvable. . The basic 2 stroke problem is oil burning re emissions. I envisage with improved charging via better porting and other changes from S.S. traditional design (see attached), can open opportunities for total supercharging independent of the crankcase to allow full re-circulating oiling. Switching then to turbo may then be a dual possibility. (via Riedel logic). The one big advantage with a short stroke version is the large gain in con rod leverage for both rods on ignition @ TDC compared to the minute gain from an offset gudgeon pin in all ordinary engines. Cylinder location in relation to rotational direction also becomes critical to gain that extra leverage on ignition and a longer power stroke for the master piston and con rod. Other gains and limitations apply to the lesser con rod's performance because of the piston trailing effect.....by about 10 to 15 mm.

 

[tech99]

I read that supercharged 2-cycle engines were used for submarines by Germany in WW1. These were very efficient and designed to run at fixed speed. By changing a step-up gear ratio between turbine and compressor, the engine output is progressively increased. However, the pumping losses eventually reduce the power available from the engine shaft. Ultimately the turbocharger is acting as a gas turbine and the engine is acting as the combustion chamber. In this situation, more power is available from the turbo shaft than from the engine shaft.

 

[Kaseytoo]

I believe I need full supercharging independent of the crank case to start and run this engine. 2 factors involved. 1.Full internal lubrication is the intention. 2. Powered supercharging needed anyway to start it anyway, so go all the way with it. Turbo may be out of the question due to overheating of the exhaust piston. An elecctro-mechanical supercharger may be best for general use.

 

[member 769127]

[Mentor Note: stewie's new thread start merged into this existing thread.]

I found this thread https://www.physicsforums.com/threads/turbocharging-carbureted-petrol-2-stroke-engines.993418/ And I found it truly fascinating — it seems that a very knowledgeable member of the discussion, who’s also generous in sharing their information, shared their experience with turbocharging two-stroke gasoline engines.

Now, I’d like to put the theory I’ve learned—by reading here and there on the internet—up for discussion and under your scrutiny. My reasoning is based on a simple single-cylinder, two-stroke gasoline engine without a reed valve, rotary valves, or anything like that—just a basic carbureted engine with the carburetor connected directly to the intake side in its naturally aspirated form.

Now, let’s suppose we want to turbocharge it using a turbocharger, and we place the carburetor before the turbo (in a draw-through configuration). We then adjust the jetting, ignition timing, and compression accordingly. After that, we install a properly tuned expansion chamber exhaust and connect it to both the hot side of the turbo and the cylinder’s exhaust port.

Here’s the statement I’d like to analyze: in this configuration, we would get both higher fresh air pressure on the intake side and higher pressure on the exhaust side due to the increased amount of gases produced. So, it would be like running the entire engine in an atmosphere denser with oxygen, thereby increasing power.

In practical terms, it would be very difficult to perfectly tune everything, so surely some fresh, unburned mixture would end up in the exhaust anyway. But do you agree with me that, despite this, we would still see a significant performance increase?
In this topic, it’s easy to go off-topic and lose the thread of the discussion. I kindly ask, if you feel like responding, to please stay on topic and consider only the theoretical engine model I described in that specific configuration, without any additional modifications. If you want, we can discuss those in other threads.

The goal here is to reach a conclusion on this fascinating subject! Thanks to everyone for your collaboration—I hope to bring some interest to this topic.

stewe

 

[bluechipx]

The very first experience with turboing a two stroke was at a snowmobile grass drag. I was still like 99% of people that "knew" turbo'ing a two stroke wouldn't work because any boost pressure would simply exit the last to close exhaust port. There was a race sled with a turbo mounted at the tail end of the expansion chamber and I inquired if he did some major mods to the engine like valves, reeds or some type of hard to do modification. He stated that there were no mods done to the engine at all. The turbo was simply mounted to the end of the expansion chamber.

I remember there were turn buckles instead of springs to hold the much greater chamber pressure from blowing the pipe connections apart. I told him it couldn't work and he asked if I just got to the race and I replied that I did. He told me to go to the starting line in ten minutes. As he was waiting for the starting signal I was still doubtful, then the next few seconds were life changing. The class he was running was open fuel, any cc, any mods and any fuel and it was a multi state event. I was stunned and all the way home on a three hour ride I quietly thought about how it was possible, then in an instant it hit me of why it worked and I almost had to pull the car over and compose myself. I wound up buying the very machine and began building an outboard turbo for my hydroplane.

I did a draw through, simply put the turbo on a plate at the exhaust side and ran a tube to the carbs former location. I own a Stuska dyno and the engine had an original 40.6 hp, the very first run on the dyno showed 85 hp and a simple turn of a screw put it a couple of hp over 100. No complicated formulas or design, it simply worked far beyond my expectations first time.

I have never done any mod to any engine, expansion chambers, nitrous, alcohol that was WAY above my expectations. 40.6 hp was good for 79-80 mph on my hydro and with the turbo I have to start easing off the throttle at 105 due to dangerous handling. As for turbo lag, a friend was running a larger, higher hp engine than me (50% more cc's and 50% more hp) and could beat me before turboing. by quite a lot. He then put a 50 hp nitrous system set-up on his rig and it really came to life. When we lined up side by side at about 40 mph and both hit it, he would get about 3/4 of a boat length on me (13 ft hydro) and I'd start coming back and passing him with much more top end than him.

Everybody seems to over complicate turbo'ing with exotic ideas, fancy jetting and several other things, just try for the right size turbo, on the smaller side if possible and put it on and watch how good it works first time! When I did my first run on the water I was just out of sight from the spectators and when I returned one of them said "it must have worked, he has an ear to ear smile". I told them I just about pulled the steering wheel out of the dashboard!

 

[member 769127]

The very first experience with turboing a two stroke was at a snowmobile grass drag. I was still like 99% of people that "knew" turbo'ing a two stroke wouldn't work because any boost pressure would simply exit the last to close exhaust port. There was a race sled with a turbo mounted at the tail end of the expansion chamber and I inquired if he did some major mods to the engine like valves, reeds or some type of hard to do modification. He stated that there were no mods done to the engine at all. The turbo was simply mounted to the end of the expansion chamber. I remember there were turn buckles instead of springs to hold the much greater chamber pressure from blowing the pipe connections apart. I told him it couldn't work and he asked if I just got to the race and I replied that I did. He told me to go to the starting line in ten minutes. As he was waiting for the starting signal I was still doubtful, then the next few seconds were life changing. The class he was running was open fuel, any cc, any mods and any fuel and it was a multi state event. I was stunned and all the way home on a three hour ride I quietly thought about how it was possible, then in an instant it hit me of why it worked and I almost had to pull the car over and compose myself. I wound up buying the very machine and began building an outboard turbo for my hydroplane.
I did a draw through, simply put the turbo on a plate at the exhaust side and ran a tube to the carbs former location. I own a Stuska dyno and the engine had an original 40.6 hp, the very first run on the dyno showed 85 hp and a simple turn of a screw put it a couple of hp over 100. No complicated formulas or design, it simply worked far beyond my expectations first time. I have never done any mod to any engine, expansion chambers, nitrous, alcohol that was WAY above my expectations. 40.6 hp was good for 79-80 mph on my hydro and with the turbo I have to start easing off the throttle at 105 due to dangerous handling. As for turbo lag, a friend was running a larger, higher hp engine than me (50% more cc's and 50% more hp) and could beat me before turboing. by quite a lot. He then put a 50 hp nitrous system set-up on his rig and it really came to life. When we lined up side by side at about 40 mph and both hit it, he would get about 3/4 of a boat length on me (13 ft hydro) and I'd start coming back and passing him with much more top end than him. Everybody seems to over complicate turbo'ing with exotic ideas, fancy jetting and several other things, just try for the right size turbo, on the smaller side if possible and put it on and watch how good it works first time! When I did my first run on the water I was just out of sight from the spectators and when I returned one of them said "it must have worked, he has an ear to ear smile". I told them I just about pulled the steering wheel out of the dashboard!

Thank you again bluechips for share your interesting experience about that!! This Is a very underrated argument but It has potential!!!
So what about the config ive wrote do you think it can make more power than n/a??

 

[bluechipx]

I'm not sure I am following your idea, you wrote;

Now, let’s suppose we want to turbocharge it using a turbocharger, and we place the carburetor before the turbo (in a draw-through configuration). We then adjust the jetting, ignition timing, and compression accordingly. "After that, we install a properly tuned expansion chamber exhaust and connect it to BOTH the hot side of the turbo and the cylinder’s exhaust port"

Clarify the last sentence please.

 

[member 769127]

Here’s what I mean: I would like to clarify once and for all whether a turbocharger can work on a basic two-stroke engine without a reed valve, rotary valve, or exhaust valve. Let’s assume we have an engine with the following characteristics:

1. A properly sized carburetor positioned in front of the turbocharger, in a draw-through configuration;
2. A suitably sized turbocharger with a self-lubrication system;
3. An intake duct running straight from the turbo to the crankcase, with no type of valve (no reed valve or anything else) and no system capable of altering the flow;
4. A single-cylinder two-stroke engine (displacement is not considered for now — let’s assume everything is correctly sized);
5. A properly tuned expansion chamber exhaust.
6. A CDI unit with properly set fixed ignition timing, and a reduced compression ratio to prevent detonation.

I'm basing this on the concept that, if everything is tuned as precisely as possible, it would be like having the entire engine operating in an atmosphere richer in oxygen, since everything is under pressure and the turbo itself acts as a sort of 'equalizer'.

So, setting aside the fact that some fresh mixture will inevitably escape through the exhaust port — because achieving intake pressure perfectly equal to exhaust pressure is purely theoretical — would we still see a performance increase? In your opinion, is this concept fundamentally flawed, or have I more or less understood how it works?


I hope to bring some clarity to this topic — or at least spark some curiosity! Thanks to everyone who takes the time to respond, and special thanks to Bluechips!

 

[bluechipx]

I feel that you would have difficulty starting the engine the way it looks. The crankcase with reeds acts like a one way valve. I put a fitting in my crankcase that monitored only the positive pressure in the crankcase by putting a reed in the line to the gauge and saw a pressure of 7 1/2 psi at high rpm's. 7 1/2 psi is enough to force a fresh charge to the cylinder and force exhaust out. With a turbo I saw over 25 psi in the crankcase. Then I also put a pressure gauge in the area between the engine and the turbo and it read 18 psi. The difference between 25 psi on one side and 18 psi on the other is still enough to clear the exhaust with a fresh charge, but as the piston clears the exhaust port on it's upward stroke, there is 18 psi in the cylinder and also to the turbo. The true boost is actually the pressure in the exhaust system that continues into the cylinder. If you have pressure gauges just after the turbo and also in the exhaust system you can really see what is happening.

 

[bluechipx]

I used an Aerodyne turbocharger that had precision high speed bearings that were oiled with a wick from a small oil reservoir. Instead of a waste gate it had several airfoil style blades in a circle around the driven turbine wheel that could be adjusted to direct the exhaust gases at different angles to control the amount of boost. There is no real reason for eliminating reeds or rotary valves with the exception of short reed life from splitting or cracking at higher boost levels. I had a pressure gauge in the tube between the turbo and the original carb position, a gauge monitoring the positive crankcase pressure and a gauge between the exhaust side of the turbo and the engine. This gave me every thing I needed to know about what was happening in the engine. I tried draw through, blow through and Haltech fuel injection and strangely the draw through performed the best.

 

[member 769127]

Thanks again for replying. What you said is clear. The reason I mentioned in the previous message that there were no valves of any kind on the intake was to try and understand whether it would actually be possible to supercharge even a basic two-stroke engine — just to finally put an end to the eternal debate: "Does 2 stroke turbo work or not?"

 

[member 769127]

It's truly a shame that this topic is so often treated superficially on the internet. If you look around, you’ll find only a few interesting videos where someone actually studies or seriously experiments with turbocharging a two-stroke engine. Unfortunately, in most cases, it's always dismissed as nonsense or a waste of time.

Only within the snowmobile and Yamaha Banshee enthusiast communities can you find anything genuinely interesting. In the motorcycle or small engine world, it's mostly just a few YouTubers doing small experiments — or others who throw a turbo on randomly with no real logic behind it.

Below, I'm sharing two very interesting links — the first one is about a fascinating turbocharged two-stroke prototype.
The second one shows the incredible power achieved by a turbocharged 125cc two-stroke engine. For anyone interested, I highly recommend watching it — and if you feel like it, let's discuss it together!

 

[bluechipx]

I left comments on the videos you sent. It is hard to believe that so many people for so many years can't get it through their heads about the exhaust pressure is the key. Myself for included for a couple of decades! As far as your design not looking like it would be able to start but should work when running, a guy I know had a design similar that he planned on using a leaf blower to start it.

 

[Baluncore]

It is a single-shaft, gas turbine engine, with a hybrid combustor.
Air is compressed, then fuel is added,
that mix passes into a cyclic cylinder combustor, with spark-ignition,
energy is extracted by the piston, to the crankshaft and flywheel,
the exhaust drives the turbine, which is on the compressor shaft.

 

[bluechipx]

Long shot here but roughly where are you located?

 

[Baluncore]

Long shot here but roughly where are you located?

@bluechipx Who?

 

[bluechipx]

Baluncore

 

[Baluncore]

At the far end of the world, on an Australian island called Tasmania.

 

[bluechipx]

I guess a visit is out then!